Once you have read the information below on the Science Team's mission objectives for Deep
Impact, take a look at Mission Results and see what they
learned.

Pre-Encounter: Science Objectives for the Deep Impact Mission

Despite all the observations of comets over the centuries and what we have learned from those
observations, there is still much that we do not know.

Ray Brown, science writer at the University of Maryland writes about the
science objectives of the Deep Impact mission and explains how the activity of making a
crater in Comet Tempel 1 will lead to information to meet those objectives.

Comet Facts

What We Still Don't Know

Comets have the most primitive, accessible material in the solar system.

We do not know what is hidden below the evolved surface layers.

Comets must become dormant.

We do not know whether ice is exhausted or sublimation is inhibited.

There must be many dormant comets masquerading as asteroids.

We do not know how to identify these bodies.

We know more chemical and physical details than for other small bodies.

We do not know how to use these details to constrain our models for comets.

Abundance of gases in the coma is widely used to infer the ices in the protoplanetary disk.

We do not know the relationship between coma abundances and those in the nucleus.

Comets break apart under small stresses.

Nothing is known about variation of material strength with scale.

The aim of Deep Impact is to answer as many of these questions as possible:

Where is the pristine material in comets?
Deep Impact's primary scientific theme is to understand the differences between the interior of a
cometary nucleus and its surface. Cometary scientists are convinced, albeit on the basis of little or
no observational data, that the surface layers of the nuclei are highly evolved. Numerous perihelion
passages can lead to significant loss of ice from the outer-most layers; if comets are as porous as
they are generally presumed to be, then this can lead to significant changes in the ice.
Calculations by Prialnik and Mekler (1991), by Benkhoff and Huebner (1995), and by Klinger (1996),
for example, all indicate that evolutionary effects are important at depths below one meter.

These models disagree on the depth of evolutionary effects (how deep the mantle will
be) and predict opposite density gradients (what part of the mantle is denser).

Do comets lose their ice or seal it in?
It is thought that as many as half the near-Earth "asteroids" are actually dormant or
extinct comets. There is clear evidence for dormancy or extinction of comets, but we have no way to
be able to choose whether a comet is dormant (ice still present but unable to escape) or extinct (ice
totally exhausted). We do not know whether the development of a mantle chokes off sublimation,
thus trapping considerable amounts of ice in the interior, or if the mantle remains quite porous so
that subsurface ice can sublime and escape freely. When the impactor makes a crater on the comet
we will be able to determine dormancy or extinction. In the case of dormancy the impact will return the
area to activity, whereas in the case of extinction no activity will be resumed.

What do we know about crater formation?
Crater growth rate and final crater morphology from the experiment will provide important clues for
the nature of the upper surface of the comet. Laboratory experiments at hypervelocities reveal that
the ejection angle depends on the porosity of the target: Highly porous materials result in higher
angle trajectories (relative to the surface).